It is generally known that standard types of stainless steels have limited galling resistance. In many commercial applications requiring stainless steel, lubricants cannot be used to prevent galling of the steel surface. For example, in the food processing industry contamination concerns prohibit the use of any lubricants to prevent galling. In response to these concerns, several galling resistant stainless steel alloys were developed having superior galling resistance compared to conventional austenitic stainless steels. Two specialty galling resistant stainless steels, sold under the trademarks Nittonic 60.RTM. and Gall-Tough.RTM., have high threshold galling stress values (TGS), nominally 7 ksi (48 MPa) and 15 ksi (103 MPa), respectively. U.S. Pat. No. 4,039,356, Schumacher et al., describes the galling resistant austenitic stainless steel alloy sold under the trademark Nitronic 60.RTM. (registered trademark of Armco, Inc., Middletown, Ohio). That alloy consists essentially of, in weight percent(%):
______________________________________ % ______________________________________ C 0.001-0.25 Mn 6-16 Si 2-7 Cr 10-25 Ni 3-15 Mo 4.0 max. N 0.001-0.4 Cu 4.0 max. Fe Bal. ______________________________________
U.S. Pat. No. 4,814,140, Magee, Jr., assigned to Carpenter Technology Corp., assignee of the present application, describes a galling resistant austenitic stainless steel alloy sold under the trademark Gall-Tough.RTM. (registered trademark of Carpenter Technology Corp., Reading, Pa.). That alloy consists essentially of, in weight percent:
______________________________________ % ______________________________________ C 0.25 max. Mn 2.0-7.0 Si 1.0-5.0 Cr 12-20 Ni 2.0-7.75 Mo 3.0 max. N 0.35 max. Cu 3.0 max. Fe Bal. ______________________________________
The austenitic stainless steel alloys described in Schumacher, et al. and Magee, Jr. provide galling resistance that is superior to the standard types of austenitic stainless steels. The alloys disclosed and claimed in Schumacher et al. and Magee, Jr. provide general corrosion resistance comparable to AISI Type 304 stainless steel. That level of corrosion resistance is adequate for use in many chloride-containing environments. However, some applications, such as valve components in the petrochemical industry, require galling resistance that is superior to conventional austenitic stainless steels and chloride corrosion resistance, especially pitting resistance, that is at least as good as that provided by AISI Type 316 stainless steel.
Type 316, an austenitic stainless steel, has very good chloride pitting resistance, but its galling resistance is much lower than the alloys described by Schumacher, et al. and Magee, Jr. Known austenitic stainless steels such as UNS S31700 and S21000 also provide good pitting resistance, but do not have the desired combination of galling and pitting resistance necessary for petrochemical applications.
It is therefore an object of the present invention to provide a stainless steel alloy which provides a superior combination of galling resistance and chloride corrosion resistance compared to known stainless steels such as Type 316, Gall-Tough.RTM., or Nittonic 60.RTM..
Some commercial applications require a galling resistant stainless steel which can be successfully formed by an upset process, such as cold or warm heading, and which remains substantially non-magnetic after such processing. For example, stainless steel fasteners such as bolts and nuts are usually formed by a cold or warm heading process. When such fasteners are used for certain computer and electronic applications, the fasteners must have good galling resistance and must be substantially non-magnetic.
One known, galling resistant, austenitic stainless steel, Gall-Tough.RTM. is known to form deformation-induced martensite when worked by an upset process. The presence of a significant amount of martensite in such an alloy greatly increases the alloy's magnetic permeability, thereby rendering these alloys unsuitable for certain computer and electronic applications. Further, the presence of a significant amount of deformation-induced martensite in a galling resistant, austenitic, stainless steel alloy greatly increases the alloy's hardness, indicating a high work hardening rate, thereby reducing its utility in products formed by cold or warm heading.
One known stainless steel, UNS S30430, is designed specifically to resist the formation of deformation-induced martensite during the heading process but has less than desirable galling resistance. It would be highly desirable to have an austenitic, galling resistant, stainless steel with superior resistance to the formation of deformation-induced martensite, compared to known austenitic, galling resistant stainless steels.